1. Field of the Invention
This invention relates to a pressure detecting circuit for a semiconductor pressure sensor and, more particularly, to compensation for the temperature-dependency of a zero point output voltage of a pressure sensor.
2. Description of the Related Art
FIG. 10 of the accompanying drawings is a perspective side view showing an overall structure of a semiconductor pressure sensor making use of the pieso-electric effect. In FIG. 10, a semiconductor pressure sensor 100 has a sensor section which is contained in a package formed of a base 101 and a cap 102. A space 103 in the package is a vacuum. The sensor section 104 is composed of a glass base 105 and a semiconductor sensor chip 106 fixed thereto, and secured to the base 101. The central portion of the sensor chip 106 is formed in a thin-plate shape on its lower side. The pressure to be detected is transmitted to the base 101 and the glass base 105 and introduced, by a pressure introductory hole 108 passing through and extending the center thereof, into a space 106a between the glass base 105 and the semiconductor sensor chip 106.
The thin-plate shape portion at the center of the sensor chip 106 deforms in accordance with a pressure difference between the spaces 106a and 103. The distortion generated by the deformation of the central portion of the sensor chip 106 is measured by a pressure detecting circuit located at this central portion, which will be described later, for detecting the pressure. A signal obtained from the pressure detecting circuit is introduced to the outer portion of the package by a lead 110 passing through and extending over the wire 109 and the base 101.
FIG. 11 is a perspective view schematically showing a structure of an upper surface of the semiconductor sensor chip 106. A bridge circuit is located at the central portion 106b of the sensor chip 106 for detecting the pressure using the piezoelectric effect. A signal processing circuit is located at a peripheral portion 106c of the bridge circuit. These circuits are formed by using methods such as impurity diffusion etc. on the sensor chip.
FIG. 12 shows a conventional pressure detecting circuit on the semiconductor sensor chip 106. In FIG. 12, the numeral 1 designates a bridge circuit in contact with a pressure detecting means at the center of the sensor chip. The bridge circuit is connected such that gauge resistors 2A, 2B, 2C and 2D form a Wheatstone bridge circuit. In practice, these gauge resistors 2A, 2B, 2C and 2D are arranged as shown in FIG. 11 to form a rectangle and extend in the same direction. The numerals 3A, 3B, 3C and 3D designate four terminals in the bridge circuit 1, in which the terminal 3A is a connecting point between the gauge resistors 2A and 2C, the terminal 3B is a connecting point between the gauge resistors 2A and 2B, the terminal 3C is a connecting point between the gauge resistors 2C and 21), and the terminal 3D is a connecting point between the gauge resistors 2B and 2D. The terminal 3A is coupled to a power source 4 while the terminal 3D is coupled to ground 5. The numerals 6 and 7 designate, respectively, a signal processing circuit for amplifying an electric potential generated between the terminals 3B and 3C of the bridge 1 and a resistor coupled between the terminal 3B and 3D and having a low temperature coefficient for temperature compensation.
The operation of the above-mentioned apparatus will now be described. The bridge circuit 1 is composed such that a voltage in accordance with a distortion voltage generated due to the applied pressure arises between the terminals 3B and 3C. Namely, when a distortion is applied to the gauge resistors 2A, 2B, 2C and 2D, their resistance values correspondingly vary. However, since the directions of receiving the distortion are different for contiguous resistors, there would arise a difference in the resistance value. Namely, for some resistors the resistance value would increase while for others it would decrease, so as to cause an imbalance in the bridge circuit 1, thereby generating a voltage. EQU Vod=kf (1)
where Vod: difference voltage between the terminals 3B and 3C, f: distortion stress, and k: proportionally constant. According to the equation (1), when the distortion stress is zero, i.e., f =0, the difference voltage would be Vod=0. In practice, however, Vod.noteq.0. If this is assumed as a zero point output voltage Voffset, tile equation (1) would be represented as follows: EQU Vod=kf+Voffset (2)
In general, the correction of Voffset is carried out by applying a corresponding suitable external correction voltage to the signal processing circuit 6. The temperature properties of Vod can be calculated by differentiating the equation (2) with the temperature. EQU .differential.Vod/.differential.T=.differential.kf/.differential.T+.differe ntial.Voffset/.differential.T (3)
if f=0, this could be represented as follows: EQU .differential.Vod/.differential.T=.differential.Voffset/.differential.T (4)
This is the temperature dependency of the zero point output voltage, which is generated by an imbalance of the temperature coefficients of the gauge resistors 2A, 2B, 2C and 2D or a residual stress generated during the assembling operation, in the case of the bridge circuit 1 for detecting the distortion. In general, the temperature dependency can be approximated by a first order function with respect to the temperature: EQU .differential.Voffset/.differential.T=.alpha. (5)
where a stands for a proportionality constant. The relationship between the zero point output voltage Voffset and the temperature T is shown in FIG. 13. In FIG. 13, the horizontal axis represents the temperature T while the vertical axis represents a zero point output voltage Voffset. If it is assumed that the available temperature range for the bridge circuit 1 (semiconductor pressure sensor) is T1-T2, this is represented by the following equation: EQU .alpha.(T2-T1)=Vofdrift (6)
Therefore, the temperature dependency of the zero output voltage in FIG. (6) should be compensated.
When the resistance values of the resistors 2A, 2B, 2C and 2D are represented by RA, RB, RC and RD respectively, and the voltage of the power source 4 is represented by Vr, the potential V3B of the terminal 3B can be expressed as follows: EQU V3B=Vr.times.[RB/ (RA+RB)] (7)
Further, the potential V3C of the terminal 3C can be expressed as follows: EQU V3C=Vr.times.[RD/(RC+RD)] (8)
Therefore, the output voltage Vod of the bridge circuit 1 can be expressed as follows: EQU Vod=Vr.times.{RB/(RA+RB)-RD/(RC+RD)} (9)
If it is assumed that the temperature coefficients of transistors 2A, 2B, 2C and 2D are the same and designated by character .gamma., and the resistance value at T=0 is Ro, it is represented as follows: EQU RA=RB=RC=RD=RO (.gamma.T+1) (10)
For correcting Vofdrift, the resistor 7 is coupled between the terminals 3B and 3D. Namely, when the resistor 7 is coupled in parallel with the resistor RB, the equation (9) can be represented as follows: EQU Vod=Vr [(RB//RP)/{RA+(RB//RP)}-1/2)] (11)
However, the character Rp designates the resistance value of the resistor 7. If the equation (10) is substituted for the equation (11): EQU Vod=-[Ro(.gamma.T+1)/[2Ro(.gamma.T+1)+4Rp}].multidot.Vr (12)
Here, if Rp&gt;&gt;RB: EQU Vod.div.-[Ro(.gamma.T+1)/4Rp].multidot.Vr (13)
The varied amount Vcomp when the range of the temperature T is T1-T2 would be as follows: EQU Vcomp=-(Ro.gamma./4Rp).multidot.Vr (T2-T1) (14)
Therefore, the Vcomp obtained from the equation (14) would become the correcting amount of the Vofdrift. Namely, the resistance value Rp of the resistor 7 should be determined so as to meet the following condition: EQU Vofdrift+Vcomp=0 (15)
The Rp can be obtained as a unique value by the following equation: EQU Rp=(Ro.gamma.Vr/4Vofdrift).multidot.(T2-T1) (16)
As mentioned above, for correcting the temperature dependency of the zero point output voltage, the resistor 7 should be inserted between the terminals 3B and 3D. Further, if Vofdrift is negative, the resistor 7 should be inserted between the terminals 3B and 3D, or the terminals 3A and 3B.
Here, attention should be paid to the fact that such a correction cannot be achieved unless the temperature coefficient of the resistor 7 is zero or significantly smaller than the temperature coefficient of the resistors 2A, 2B, 2C and 2D. Further, by inserting the resistor 7, the zero point output voltage would correspondingly vary, but this can be independently corrected by a separate correcting method.
Since the conventional pressure detecting circuit for a semiconductor pressure sensor has been composed as mentioned above, a resistor with quite a small temperature coefficient must be provided for correcting the temperature dependency of the zero point output voltage, which seriously hinders integrating the circuit.